"Nice penis!" the foxy brunette says to the drunken oaf urinating in the bushes at the beginning of "Cyrus," a romantic comedy that follows the genre's conventions while still feeling offbeat and insightful. Dragged to a party by his concerned ex-wife and her patient fiancé, John has allowed a steady influx of vodka and Red Bull throughout the evening to slowly erode the dam that held back his self-pitying desperation, forcing even the most kindhearted female guests to flee from his painful attempts at conversation. But not Molly. Sure, she overheard snippets of his last awkward come-on ("I'm in a tailspin"), yet Molly is nonetheless charmed by John and his refreshing honesty, much to John's sozzled shock. "Are you flirting with me?" he asks her. "I'm like Shrek!"

The boxer-faced John C. Reilly stars as John, so, yeah, compared to Marisa Tomei's Molly, he is like Shrek. (Though truthfully, up against the beyond-luminous Tomei, I too am sort of like Shrek.) In addition to her beauty, Molly is also the kind of woman who will rescue a guy with a Human League duet and leave soon after sex. But that last part actually bothers the now-smitten John, and his understandable worry that his sweet, funny catch might already have someone compels him to follow Molly home one night. Turns out there is another man in her life. As played by the increasingly versatile Jonah Hill ("Get Him To The Greek"), his name is Cyrus, and he will soon make John's fears of cuckoldry far more palatable than the reality, which is that Molly has a 21-year-old son who hasn't yet learned to share.

Written and directed by one set of brothers (mumblecore pioneers Jay and Mark Duplass) and executive-produced by another (filmmaking royalty Ridley and Tony Scott), "Cyrus" develops in an uncomfortably hilarious way as the initially welcoming Cyrus, who until now had enjoyed his mother's exclusive attention, begins to show his true colors...but, of course, only to John. When Cyrus isn't having separate conversations with John and Molly wherein he's planting seeds of doubt about their fledgling (and surprisingly fast-moving) relationship, he's silently intimidating his perceived rival with an idea cribbed from Bob Dylan's promo clip for "Subterranean Homesick Blues," or he's mouthing a barely perceptible "Fuck you" to John while hugging his mother, middle finger unfurled behind her back. John's dilemma is whether to endure Cyrus's mind games and spare Molly the truth about her manipulative son, or break Molly's heart by putting the sociopathic little weasel in his place.

Moviemaking rules dictate that when John and Cyrus agree to a temporary truce for a big event - namely the intimate wedding of John's ex Jamie, played by the generous Catherine Keener - their game of one-upmanship will reach its cathartic, slapsticky zenith and everyone will have to face some third-act truths. And though the Duplass brothers don't deviate from that rom-com blueprint, they do put their own stamp on it by employing the twitchy handheld shooting style that typified low-budget work like 2008's chatty horror comedy "Baghead" and crafting self-serving characters who behave in depressingly shortsighted ways.

The Duplasses also have no qualms about making you squirm, playing up the weird Oedipal angle ("You deserve someone who can love you in a way that I can't love you," Cyrus tells Molly) and holding shots for maybe a half-beat longer than others might, thus enabling you to fully fathom the sight of a pantsless Cyrus clutching a knife in a blankly menacing manner that could skew either comic or tragic. The genius way Hill locks eyes with Reilly during the scene where Cyrus performs his aggressive New Age-y techno for his mother's new man is so deceptively minimalist, with Cyrus's deadpan gaze conveying everything and nothing all at once.

As far as Reilly and Tomei, they're pros; Reilly is a master of the priceless reaction, and Tomei (an Oscar winner, don't forget) continues to make the most of her second act, the maternal bond between the teeny Tomei and the Weeble-shaped Hill quite believable and verging on creepy in its familiarity. But if there is a complaint to be made, it's that "Cyrus," at just over 90 minutes, seems a bit slight. Why, for instance, would a woman who hasn't had a man stay overnight in 20 years shack up with one in a matter of days? These characters, with their very human hangups, deserved more fleshing out, especially in light of the fact that we like them and we're like them.